Paper is manufactured predominantly from wood fibers as wood pulp. There are three major processes for the production of pulp: wood digestion, brown stock washing, and bleaching. Wood digestion is often referred to as “cooking” and it is the process of using chemicals to dissolve approximately 50% of the wood components facilitating the liberation of wood fibers. Brown stock washing is the process where the fibers are separated from non-fibrous wood components and spent cooking chemicals by using recycled and or clean water and filtrates from other processes as wash water. The non-fiber portion of this stream is referred to as black liquor and is sent to a separate process where the chemicals are recovered. For fine grades of paper, e.g. printing and writing, the fiber is bleached by treatments with oxidizing chemicals to “brighten” the pulp.
Brown stock washing is a critical process in pulp mill operations. Its purpose is to separate spent cooking liquors and undesired degraded wood components from the wood fibers after the cooking process using a minimal amount of wash water. The optimization of this process improves chemical recovery, mill energy balance/usage, reduces unwanted chemical consumption in downstream applications, and decreases environmental impact. When operated effectively, the brown stock washers can be a source of considerable savings to a pulp mill.
It is common for a brown stock washing system to be run inefficiently. Many of the NA operating pulp mills are antiquated and use original process equipment instead of upgrading to newer more efficient washers. A global problem is that due to the complexity of a washing process the operator's are sometimes uncertain of how to correctly respond to unfavorable wash conditions.
Many economically feasible chemical additives have been found to influence the operation of brown stock washers in a positive way. By using filtration theory and how it applies to a washer, it may be possible to better understand how certain processes and chemicals influence a fiber washing process.
Many technologies exist for washing cellulosic fibers, but each design incorporates the same basic principles. These basic principles are dilution and displacement washing. Dilution washing is described as diluting a cellulosic material with cleaner filtrates and wash waters then removing the filtrates and wash waters through a filtration separation process under vacuum or pressure. Displacement washing is done by replacing the liquid portion of cellulosic slurries with cleaner filtrate or wash water under vacuum or pressure. Dilution washing is more effective, but displacement washing uses less water. Typical equipment for washing cellulosic slurries incorporates both dilution and displacement washing, or a series of displacement washing zones. Cellulosic washer designs include perforated drums, porous wires, extraction plates, and screw presses, all either under vacuum, under pressure, or used to form nips for pressing.
Organopolysilicone defoamers functionalized with polyether have been used, mainly, to aid in emulsifying and dispersing defoamer in incompatible mediums. The addition of hydrophobic silica particles in the formulation is mandated by the fact that they play an important roll, along with other ingredients, in foam suppression. The following patents disclose the use of polyether functionalized silicone copolymers, mainly a mixture of polydimethylsiloxane and polydimethylsiloxane-co-polyhydromethylsiloxane copolymers with various molecular weights, as good defoaming agents. The majority of patents use hydrosilylation as a mean to incorporate allyl-functionalized polyethers onto silicone copolymer backbones possessing randomly alternating dimethylsiloxane and methylhydrosiloxane monomer units.
Dow Corning Co. (U.S. Pat. No. 9230049.9) disclosed curable liquid organopolysilicone compositions dispersed in a liquid continuous phase in the form of an emulsion. The curable liquid comprised of organopolysilicone blends of diorganopolysilicone possessing a silicone-bonded hydroxyl group and/or alkoxy group having one to six carbon atoms and silicon resins containing a hydrolyzable functionality. In Kulkarni et al. (U.S. Pat. No. 4,509,532), the viscosity of the dimethylpolysiloxanes oil was in the range of from 5000 to 30,000 cS at 25° C., which improved efficiency in difficult to defoam aqueous systems. Keil et al., (U.S. Pat. No. 3,784,479) disclosed a foam control composition of a base oil selected from polyoxyprolene polymers, polyoxyproylene-polyoxyethylene copolymers or silicone-glycol copolymers combined with dimethylpolysiloxane and silica filler. In another closely related U.S. Pat. No. 3,984,347, Keil disclosed foam control compositions which consisted mainly of base oil selected from polyoxypropylene polymers, polyoxyproylene-polyoxyethylene copolymers or silicone-glycol copolymers, a foam control agent comprising a liquid dimethylpolysiloxane, silica filler, and a silicone copolymer dispersing agent. Similar composition containing higher molecular weight polydimethylsiloxane was reported by Schiefer (U.S. Pat. No. 4,762,640) was also shown to be useful in defoaming of highly acidic aqueous systems. Aizawa et al., (U.S. Pat. Nos. 463,489 and 4,749,740) disclosed a method for producing silicone defoamer compositions containing a mixture of filler, silicone resin and a catalyst to promote reaction of the components at 50° C. to 300° C. Starch, in U.S. Pat. No. 4,983,316, disclosed silicone antifoam emulsions particularly suited for controlling foam in aqueous detergent systems. In this patent, the above mentioned compositions of Aizawa et al. are used in conjunction with a secondary silicone antifoam agent dispersed in a polypropylene glycol continuous phase. McGee et al. (Aus. Pat. No. 34059/89), by combining the above mentioned compositions of Aizawa et al. with particular silicone glycol compounds provided improved antifoams for use in high pH aqueous systems, particularly pulp mill liquors.
Dow Corning Corp. (EUR Pat. No. 116-7502B1), disclosed silicone-based foam control compositions where silicone antifoam/silica particles were dispersed in a detergent compatible carrier. The composition consisted of a silicone-based antifoam agent made from cross-linked silicone and branched silicone fluid. In another patent Dow Corning Corp. revealed the former formulation that used vinyl functionalized MQ resin, cross-linked polymethylhydrosiloxane fluid with a viscosity of 17,000 cSt. methylhydrosiloxane, and vinyl-terminated polymethylhydrosiloxane (450 est) were cross-linked using a platinum catalyst. Dow Corning Corp. (EUR Pat. No. 1167456B1) revealed another formulation by using the same highly cross-linked poly-organosiloxane fluid described in (U.S. Pat. No. 4,749,740), adding mineral oil as another component in the formulation.
Recently, a method for preparing a composition similar to that described by Aizawa et al. was disclosed by Miura (U.S. Pat. No. 5,283,004). It is disclosed that all the ingredients, including a catalyst, must be reacted at elevated temperatures to obtain the desired antifoam agent. John et al. (U.S. Pat. No. 217,501) disclosed a foam control composition that improved performance in high foaming detergent compositions. The defoamer comprised (1) a liquid siliconesili having a viscosity of at least 7×10-3 m2/s at 25° C. and prepared by mixing and heating a triorganosiloxane end-blocked polydiorganosiloxane, the polydiorganosiloxane having at least one terminal silanol group, and an organosiloxane resin comprised of monovalent and tetravalent siloxy units that have at least one silanol group per molecule, and (II) finely divided hydrophobic fillers, McGee et al. (U.S. Pat. No. 5,380,464) disclosed a foam control composition containing a silicone defoamer and a silicone glycol copolymer which is particularly effective in defoaming highly acidic or highly basic aqueous systems. Union Carbide Corp., (EUR Pat. No. 273-4482) edify a foam suppressant composition made by a free radical polymerization of a mixture of diorganopolysilicones, silica particles, and a free radical initiator. A European patent (EUR Pat, No. 0285391) disclosed organopolysilicone emulsions comprising a hydroxyl end blocked organopolysilicone, an aminoxy group containing organosilicon compound, a surfactant, and water. Dow Corning Co. (U.S. Pat. No. 6,207,722 B1) disclosed a defoamer composition made from polysiloxanes and a resin-filler prepared from vinyl-functionalized silanol, trimethylsilyl functonalized silicone resin (MQ resin). Elms et al. (U.S. Pat. No. 6,512,015 B1; Jan 28, 2003; Dow Corning Co.) disclosed a foam control composition prepared by mixing trimethylsiloxane-terminated polydimethylsiloxanes hydroxyl-terminated polydimethylsiloxanes polysilicate resin, and a catalytic amount of potassium silanolate to promote cross-linking between the added components. In addition to this formulation, Silwet L-77® silicone glycol, L-540 silicone polyglycol having block copolymer of PO:EO ratio 50/50, and mineral oil was added. Fey at al. (U.S. Pat. No. 5,908,891) disclosed a dispersible silicone composition comprising (I) a silicone prepared by reacting a trimethylsiloxane-terminated polydimethylsiloxane, a hydroxyl-terminated polydimethylsiloxane, hydrophobic silica particle, and a catalytic amount of alkali metal silinolate to promote the reaction of the other components and (II) mineral oil. Fey et al. further discloses that the mineral oil is effective as a dispersing agent for the silicone composition (I). Willibald et al, (US Pat. Application No. 2011/0021688 A1) owned by Wacker Chemie AG, issued in Aug. 21, 2008, disclosed defoamer formulations using trimethylsiloxane-terminated polyhydromethylsiloxane mixed with allyl polyether with PO/EO ratio of 4.0. The allyl polyether was hydrosilylated onto PHMS using a platinum catalyst at 100° C. The resulting polymer surfactant had a viscosity of 870 mm2/s (25° C.). The polymer was further cross-linked with hexamethylene diisocyanate followed by the addition of Emulan® HE 50 (BASF) to give a final copolymer solution that has a viscosity of 2100 at 25° C. A defoamer composition (EUR Pat. No. 0638346) was disclosed that comprised of a reaction product prepared by heating a mixture of a polyorganosiloxane fluid, a silicon compound, hydrophobic silica particles, and a catalytic amount of potassium silanolate (for promoting the reaction of the other components) at a temperature of 50° C. to 300° C. The patent further disclosed the use of the nonionic silicone surfactants Silwet L-77 and L-540, and hydrophobic silica particles in the final compositions. Kremer, (US Pat. No. 2005/01019675 A1) disclosed a defoamer composition consisting of polymethylhydrosiloxane resins cross-linked with alkyl silicates emulsified in kerosene. This formulation is claimed to require lower amounts of silicone. The final composition contains 90% kerosene and 8% 600,000 centistokes of a cross-linked linear polymethylhydrosiloxane and 2% of a cross-linked branched polydimethylsiloxane and alkyl-silicate.